The present invention relates to a biaxially oriented polyethylene-2,6-naphthalate film for capacitor dielectric.
With miniaturization of electric and electronic devices, necessity is noted for reduction in thickness of plastic film for capacitor dielectric.
Polyethylene terephthalate film and polypropylene film have been predominantly used in this field, but attention has focused recently on polyethylene-2,6-naphthalate (hereinafter abbreviated as PEN) film for its high heat resistance, high mechanical strength and easiness of reduction of thickness.
PEN film, however, has the problem that when the film thickness is reduced, especially when it is reduced to 2.0 .mu.m or less, the slip characteristics of the film become lower and work ability is deteriorated. For solving this problem, a method is employed in which fine particles are blended in the film. However, the presence of such particles in the film often leads to deterioration of electric properties of the film.
Generally, inorganic particles have poor compatibility with polyethylene-2,6-naphthalate, so that when the film is stretched, large voids are created around the particles to deteriorate the dielectric strength of the film. Also, surface roughness of the film often becomes excessively coarse, causing a reduction of electrostatic capacity of the capacitor.